The present invention relates to a method and to an apparatus for controlling radiation power and/or field strength during reading of a magneto-optical recording medium comprising a recording or storage layer and an expansion or read-out layer, such as a MAMMOS (Magnetic AMplifying Magneto-Optical System) disk. The invention also relates to a magneto-optical recording medium for use by the method and in the apparatus according to the invention.
In magneto-optical storage systems the minimum width of the recorded marks is determined by the diffraction limit, that is, by the Numerical Aperture (NA) of the focussing lens and by the radiation wavelength. Therefore, a reduction of this width is generally based on shorter wavelength radiation and/or higher NA focussing optics. During magneto-optical recording the minimum bit length can be reduced to below the optical diffraction limit by using Laser Pulsed Magnetic Field Modulation (LP-MFM). In LP-MFM the bit transitions are determined by the switching of the field and the temperature gradient induced by the switching of the radiation source such as, for example, a laser. For read-out of the small crescent shaped marks recorded in this way, Magnetic Super Resolution (MSR) or Domain Expansion (DomEx) methods have to be used. These technologies are based on recording media with several magneto-static or exchange-coupled RE-TM layers. According to MSR, a read-out layer on a magneto-optical disk is arranged to mask adjacent bits during reading while, according to domain expansion, a domain in the centre of a spot is expanded. Because of the advantage of the domain expansion technique over MSR, bits with a length below the diffraction limit can be detected with a similar signal-to-noise ratio (SNR) as bits with a size comparable to the diffraction limited spot. MAMMOS is a domain expansion method based on magneto-statically coupled storage and read-out layers wherein a magnetic field modulation is used for expansion and collapse of expanded domains in the read-out layer.
In the above mentioned domain expansion techniques, like MAMMOS, a written mark from the storage layer is copied to the read-out layer upon laser heating by a radiation beam and by applying an external magnetic field. Due to the low coercitivity of this read-out layer, the copied mark will expand to fill the area of the optical spot and can be detected with a saturated signal level which is independent of the mark size. Reversal of the external magnetic field collapses the expanded domain. On the other hand, a space in the storage layer will not be copied and no expansion occurs.
The resolution of the MAMMOS read-out process, that is the smallest bit size that can be reproduced without interference from neighbouring bits, is limited by the spatial extent of the copy process (copy window) which is determined by the overlap of the temperature-induced coercivity profile and the stray field profile of the bit pattern which depends on the strength of the external magnetic field. The radiation power that is used in the read-out process should be high enough to enable copying. On the other hand, a higher radiation power also increases the overlap due to the fact that the coercivity Hc decreases and the stray field increases with increasing temperature. When this overlap becomes too large, correct read-out of a space is no longer possible, because false signals are generated by neighbouring marks. The difference between the maximum and the minimum allowed radiation power determines the power margin. This power margin decreases strongly with decreasing bit length. Experiments have shown that with the current read-out methods bit lengths of 0.10 xcexcm can be correctly detected, i.e. at an extremely small power margin (in the range of 1 bit of a 16 bit Digital-to-Analog Converter). Hence, a good balance of the radiation power and the intensity of the external magnetic field is an important factor when selecting optimum operating conditions.
However, even if optimum conditions have been set during an initial stage of a reading operation, the initial balance may be disturbed during reading due to environmental changes. These environmental changes comprise field blurring, disk tilt, temperature changes, non-uniformities of the thickness of the protective coat of the disk, and influences of the slider movement on the magnetic head. Thus, controlling the radiation power and the magnetic field strength during read-out is essential.
JP-A-2000-215537 discloses a method and an apparatus for controlling the radiation power and/or the field strength of the external magnetic field. In the method disclosed in JP-A-2000-215537 information defining a prescribed section on the disk and pulse information defining a prescribed pulse number are read from a specific area on the disk. Next, the number of pulses contained in the information read from the prescribed section is counted and compared with the prescribed pulse number. Based on the result of the comparison, the radiation power or the field strength is adjusted. However, this solution requires a specific type of recording record with specific pre-recorded information in specific prescribed regions. Furthermore, the control can only be performed for the prescribed regions with a given number of pulses (that is marks).
It is an object of the present invention to provide a method, an apparatus and a record carrier for providing improved power and/or field control which enables adjustment during the whole read-out process.
This object is achieved by a method as claimed in claim 1 or 2, by an apparatus as claimed in claim 13 or 14, and by a record carrier as claimed in claim 17 or 19.
Accordingly, the use of the run length characteristics for evaluation of a misbalance between the external magnetic field and the radiation power, resulting in, for example, an excessive copy window size, provides the advantage that the radiation power and/or the field strength control can be performed based on normal user data recorded on the recording medium, that is, as long as the run-length constraints of the coding of the normal user data are known. Thus, a continuous control function is provided without requiring a modified or specially pre-recorded recording medium. Moreover, the control according to the invention has the additional advantage that it is a so called xe2x80x9crunning-controlxe2x80x9d, that is the radiation power and/or the field strength control is performed while reading the user data without the need to perform a separate control step which would interrupt the reading of the user data.
According to an advantageous embodiment, a copy window size is determined in said comparison step, on the basis of which a control signal for said controlling step is generated. The copy window size is determined based on a detection of run length violations which may be determined by a pulse counting function or by a timer function. The copy window size determined may then be used to correct errors in the reading signal.
Preferably, the pulse pattern corresponds to the user data recorded on said recording medium. As an alternative, the pulse pattern corresponds to a test pattern with pre-defined mark and space run lengths recorded on said recording medium.
According to a further embodiment, the comparison step is performed based on a look-up table linking the copy window size to a corresponding number of false peaks or missing peaks in the pulse pattern.
According to a further embodiment, the control step comprises outputting a first control signal for coarse adjustment by radiation power control and a second control signal for fine adjustment by field strength control.
According to a further embodiment, a control information used in said controlling step is provided on the recording medium. This control information defines a medium-related characteristic between copy window size and radiation power data.
Other advantageous embodiments are defined in the dependent claims.